The dewaxing of hydrocarbons to produce liquid products of lower pour point is a process of great commercial significance. Although alternatives exist, the use of shape-selective catalysts, such as the intermediate pore size zeolite catalysts, to selectively convert those paraffins that contribute the most to high pour points has many advantages over other methods. Thus, catalytic dewaxing over shape-selective zeolites will likely be the most commercially significant dewaxing process in the hydrocarbon processing industry.
Catalytic dewaxing of hydrocarbon oils to reduce the temperature at which the precipitation of waxy hydrocarbon occurs is a known process and is described, for example, in the Oil and Gas Journal, pages 69-73. A number of patents have also described catalytic dewaxing processes such as U.S. Pat. No. Reissue 28,398 which uses a dewaxing catalyst comprising zeolite ZSM-5. U.S. Pat. No. 3,956,102 describes a process gor hydrodewaxing a gas oil with a ZSM-5-type catalyst. U.S. Pat. No. 4,100,056 describes a mordenite catalyst containing a Group VI or Group VIII metal used to dewax a distillate from a waxy feed. U.S. Pat. No. 3,755,138 describes a process for mild solvent dewaxing to remove high quality wax from a lube stock, which is then catalytically dewaxed to specification pour point.
Catalytic dewaxing may be applied to both distillate and lubricant feedstocks. The Mobil Distillate Dewaxing (MDDW) and Lube Dewaxing (MLDW) processes now have widespread commercial acceptance and variations on both processes have been described. These processes are described in Catal. Rev.--Sci. Eng. 28 (2&3), 185-264 (1986), especially pages 241-247, to which reference is made for a description of these dewaxing processes as well as of the MOGD process.
Improvements to the basic catalytic lube dewaxing processes include multistage dewaxing in which the lube chargestock is cascaded from a first dewaxing reactor to subsequent dewaxing reactors as described in U.S. Pat. No. 4,648,957. In the process described in this patent light products are separated from the first hydrodewaxing reactor effluent and are hydrotreated and then mixed back with the dewaxed lube before being processed in another hydrodewaxing/hydrotreating step. Other multibed hydrodewaxing processes and apparatus are described in the literature such as that described in published European Application No. 86/310083.0.
These dewaxing processes, which employ an intermediate pore size zeolite such as ZSM-5 as the effective component of the dewaxing catalyst, produce significant quantities of olefins as a consequence of the shape-selective dewaxing reactions. The olefins produced in the dewaxing reactions include both light olefins, mainly C.sub.3 and C.sub.4 olefins as well as C.sub.5+ olefins in the gasoline boiling range, as a result of which a relatively high octane olefinic naphtha is one of the by-products of the process. The reactions which take place in the dewaxing process are, essentially, shape-selective crackig of the waxy paraffinic components of the feed and although hydrogen is conventionally present to retard catalyst aging, together with a metal component such as nickel on the catalyst, the conditions employed are not conducive to saturation so that significant quantities of gasoline range and lighter olefins are produced in the process, as described in U.S. Pat. No. 4,695,364. In the dewaxing of lube boiling range hydrocarbons, as in the MLDW process, the effluent from the dewaxing reactor may be cascaded directly into a hydrotreating reactor in order to saturate and stabilize lube range olefin in the dewaxing products. Similarly, in distillate dewaxing, a hydrotreater may be provided in order to remove unsaturation unless the dewaxed distillate is combined with virgin distillate and treated together with it in the refinery CHD unit. In both these cases, however, the hydrogen consumption of the hydrocoating step is needlessly increased by the saturation of olefinic components outside the boiling range of the desired products, principally of C.sub.5- and gasoline rage olefins. To reduce hydrogen consumption it would be possible to arrange for separation between the dewaxing reactor and the hydrotreater but this may stil leave lower olefins to be carried over into the hydrotreater.
It has now been found that improved separation of the lower olefinic materials may be provided by stripping the dewaxed products prior to hydrotreating, preferably using an oil solvent such as naphtha which is fed into the top of the stripper/separator so that the recycle gas is essentially free of wet gas and heavier fractions. Operation in this manner confers several benefits. One is a significant reduction in the hydrogen consumption in the hydrotreating reactor since the majority of the light olefins are no longer hydrotreated in the hydrotreating reactor. In addition, the hydrogen circulation rate for the hydrotreating reactor can be better controlled so as to reduce the pressure drop across the hydrotreating reactor.
Alternatively, the hydrotreating reactor can be operated at a higher pressure level than the hydrodewaxing reactor; this affords the opportunity to modify the pour point of dewaxed lube products.